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The line of mice introduced in this study indicates what happens when macroautophagy is specifically blocked in dopamine (DA) neurons by deletion of Atg7, a key component of the autophagy machinery. There have been, to my knowledge, three such floxed Atg7-deficient lines published now: While there are findings indicating the existence of forms of Atg7-independent macroautophagy in some cell types, these mutant DA neurons do not appear to form autophagic vacuoles and thus cannot conduct normal macroautophagy.

The models include one from our lab that uses a dopamine uptake transporter (DAT) promoter to drive Cre expression (Hernandez et al., 2012); one from Zhenyu Yue’s lab that used a tyrosine hydroxylase promoter (Friedman et al., 2012); and the present study, which uses both a DAT and engrailed promoter. At this time, the findings from these three models appear to support each other: It appears that in younger animals, there is increased axonal outgrowth with larger synaptic terminals and greater DA release, indicating that macroautophagy normally serves to remove synaptic components, including synaptic vesicles and mitochondria. With advancing age, however, there is a buildup of organelles and other intraneuronal components that would otherwise have been degraded, which may eventually lead to neuronal death.

Overall, data from the field indicate eventual death of all cell types that cannot undergo macroautophagy: It is, after all, the only means known for cells to degrade many of their own components. Indeed, it is surprising that it takes so long for dopamine neurons to die in these mutants, particularly as the ventral midbrain dopamine neurons have the most extensive axonal outgrowth of any neuron studied—as long as half a meter in the mouse (Matsuda et al., 2009). One might guess that they would be particularly dependent on macroautophagy to maintain these extensive axonal arbors.

In this new mouse line, as expected, there is an accumulation of ubiquitinated structures that would normally be degraded, but there is also no sign of the α-synuclein aggregates that are a hallmark of nearly all cases of PD. Note that macroautophagy-based degradation of α-synuclein has long been observed (Stefanis et al., 2001). The lack of α-synuclein aggregates in old mice with no macroautophagy argues against a lack of macroautophagy per se as a cause of PD.

The neurons of all humans, including PD patients, feature an extensive buildup of autophagic vacuoles throughout life (Sulzer et al., 2008), including so called lipofuscin granules: In fact, younger and older brain tissue can be differentiated by these organelles. In substantia nigra neurons, these include prominent neuromelanin granules, and these pigmented autophagic vacuoles could interfere with delivery of components to lysosomes for complete degradation. However, those autophagic vacuoles form in these neurons in both patients and controls, suggesting that a lack of macroautophagy per se may not explain PD.

There is a theory from Richard Youle’s laboratory that some PD mutations may block the degradation of mitochondria in neurons (Lazarou et al., 2012). While a blockade of mitochondrial turnover should clearly be toxic, such a hypothesis alone cannot explain all aspects of the disease, such as the greater vulnerability of particular neurons. In that particular model, it is also not clear if problems in degradation of other cargo, such as lipid droplets, endosomes, protein aggregates such as Lewy bodies, or plasma membrane are impacted, and how this may underlie PD.

Now that the field is paying attention to the role of macroautophagy in neuronal function, there is evidence for a lack of normal mitochondrial turnover in various nervous system diseases. For example, recent work by our lab and Ana Maria Cuervo’s group indicates that in another neurodegenerative disorder, Huntington’s, there is a normal or even exacerbated level of macroautophagy, but that particular cargo, including mitochondria and lipid droplets, are not adequately recognized and so build up (Martinez-Vicente et al., 2010): This may or may not underlie neuronal death. In unpublished work, we also found evidence supporting a lack of macroautophagic turnover of mitochondria in essential tremor tissue and even in autism brain. As there are different neuronal targets for each of these disorders, it would suggest that a specific lack of mitochondrial turnover by macroautophagy by itself is unlikely to cause these diseases, and if this step is indeed required for neuronal death or disease features, there must be unknown additional mechanisms that make it specific for particular neurons.

In summary, macroautophagy is a general cellular stress response pathway in essentially all eukaryotic cells and serves to maintain their health, and so it is interesting to test the hypothesis that some diseases may be due to deficient macroautophagy. An interesting finding from both this and Yue’s study is that it takes so impressively long for a disease state to occur when macroautophagy is blocked, and that while cells do eventually die as expected, they do not show features associated with PD, such as α-synuclein aggregates. It remains possible that the lack of rapid effect is due to some form of compensatory degradative response during development of the mutants, such as increased phagocytic activity by astrocytes or microglia, but this has not been identified to date.

Perhaps one role of the increased microglia in affected regions in PD brain is to replace deficient macroautophagy, but at this time the idea is unfounded speculation. I suspect that this set of papers is quite valuable in indicating that a lack of the ability to induce macroautophagy per se is unlikely to be a cause of PD.

Impairment of autophagy-lysosomal pathways is increasingly regarded as a major pathogenic event in neurodegenerative diseases, including Parkinson’s disease (PD). This new paper from Ishrat Ahmed and colleagues emphasizes once again that autophagy is an important pathway involved in Parkinson's pathogenesis. This work adds to other studies published this year from Zhenyu Yue's (Friedman et al., 2012) and David Sulzer's (Hernandez et al., 2012) labs. The three studies generated different autophagy-deficient, conditional knockout mice with Atg7 deleted in dopaminergic neurons. The use of different tissue-specific knockouts using the Cre/lox technique makes these models similar, although not strictly identical. Mice generated by Ahmed and colleagues exhibited an age-dependent progressive loss of dopaminergic neurons, reduction in dopamine levels in the striatum, and accumulation of ubiquitinated proteins as well as the autophagy substrate p62. The same phenotypes have also been observed in the other Atg7 mutant mice. However, no α-synuclein inclusions have been observed. One plausible explanation might be that Atg7 is involved in the first steps of the autophagy-lysosomal pathway and does not alter lysosomal function. Hence, α-synuclein clearance is not directly compromised, because α-synuclein is predominantly degraded by chaperone-mediated autophagy. Nevertheless, this mouse model, with directed genetic deletion of an essential autophagy gene Atg7 in dopaminergic cells, highlights that dysfunctional autophagy may act as a contributor to PD neurodegeneration.

Besides autophagy, there is another important pathway for regulated protein degradation, i.e., the ubiquitin/proteasome pathway. It is important to consider the role played by both pathways in maintaining intracellular protein homeostasis. I would like to suggest two articles below that bring this to mind.